The object of this study is to investigate the possibility that changes in the blood volume within a region treated by radiotherapy may be used as an early indicator of response to therapy. The methodology involved requires the patient to breath air containing a trace quantity (less than 0.04 ml CO total) of carbon-11 labeled carbon monoxide (10 mCi 11CO total) and about 5 minutes later to have the activity distribution within the region of interest imaged from outside the patient using a body scanning system capable of absolute activity quantitation. These data are then processed giving a blood volume map of the tumor region and control contralateral or in other standard body region. These studies will measure lung tumor response to radiation with attempts to correlate early circulating hemoglobin volume changes with the later observation by radiographic appearance of (1) radiation pneumonitis, (2) fibrosis and (3) tumor regrowth. It may prove possible to predict the intensity of these tissue injury effects at an earlier time by observation of the intensity of the early vascular volume changes. Later increases in blood volume may prove a sensitive indicator of tumor regrowth.